It is well known to sense a force by using that force to generate hydraulic pressure which is measured to ascertain the magnitude of the force. Such devices typically include a piston in a cup with a seal on the diameter of the piston to prevent liquid leakage. A connection to the liquid filled chamber provides pressurized liquid to a pressure gage which is calibrated to read in units of force.
Semiconductor pressure sensors are manufactured in large quantities by micromachining silicon wafers. Many designs based on many technologies and many physical principles are known. Many of these sensors require additional circuitry to achieve a useful function. Typically, an integrated circuit complements the micromachined pressure sensing element. Certain of these sensors are suitable for operation submersed in liquid and operate by sensing the pressure in the liquid.
Force sensors that convert force to hydraulic pressure have the disadvantage that they are unresponsive to forces that tend to draw the piston away from the liquid and create a vacuum. To measure forces in both directions a bias force on the piston is required which has the consequence that in the case of zero applied force a substantial pressure is being measured with a potential for drift with temperature and time.
Welding and brazing are common methods of joining metals that provides good strength and sealing against leakage. Welding commonly means joining metals by melting the metals being joined at the joint with or without addition of more metal. Brazing commonly means applying a different metal at the joint that wets the metals and hardens to join the metals.
Belleville springs are washers formed to be slightly conical (dished). Certain Belleville springs are called "constant force springs" because the force required to compress the spring is approximately independent of the amount of compression over a range of compressions. Belleville springs having a dish height to thickness ratio of about 1.4 are constant force springs when they are compressed to flatness. Herein, the term "constant force spring" is defined to include Belleville springs and other springs comprising dished washers operating in a range of compressions at which the spring force or pressure the spring force causes in a fluid confined by the spring is approximately independent of the spring compression.
It is well known to reduce the number of wires required for connection to a sensor to two by the sensor simultaneously drawing power through the two wires to power itself and by drawing pulses of current over and above the current required to power itself and modulating the width or magnitude of the pulses of current to indicate the physical quantity being sensed.
A general object of this invention is to provide a force sensor that is particularly adapted for sensing force resulting from the weight of an occupant of a seat of a vehicle for informing occupant protection systems of automotive vehicles which also overcomes certain disadvantages of the prior art.